Modulation and cross-relaxation effects on the lineshape of strongly saturated nuclear magnetic resonance in solids

We have observed the nuclear magnetic resonance dispersion lineshape in aluminum, under the common experimental condition of slow passage with magnetic field modulation, while applying an rf field, HI' intense enough to produce saturation, and have found that the lineshape does not agree with the very weak HI theory of Bloembergen, Purcell, and Pound, or with Redfield's strong HI theory. We have qualitatively explained the lineshape by assuming that the spin system in the rotating frame can be divided into two parts which have unequal temperatures, and which are able to cross relax. For two limiting cases, large HI' and HI small, but still large enough to saturate, we have calculated the lineshape for all modulation frequencies, and have obtained semiquantitative agreement. The small HI calculation is based upon the theory of Provotorov and of Goldman. The large HI theory is a phenomenological extension of Redfield's theory. We have measured the dipole spin lattice relaxation time and the cross relaxation time by observing the signal as a function of modulation frequency. This method has the dual advantages of the signal to noise of steady state resonance, and the measurement of time on a frequency scale.U of I OnlyThesi